Question: The scale on a map is 1cm : 1km. If two cities are 3cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 1cm on the map corresponds to an actual distance of 1km. A distance of 3cm on the map is the same as 3 $\cdot$ 1cm. The actual distance between the two cities is 3 $\cdot$ 1km, or 3km.